1. Field of the Invention
This invention relates to a conductive paste suitable for producing an outer electrode of an electronic part, more specifically, to a conductive paste wherein glass frit contained therein contains substantially no lead oxide that is harmful to environment. Further, this invention relates to an outer electrode of an electronic part that is obtained by calcining the above-mentioned conductive paste, a process for producing the same and an electronic part equipped with the outer electrode.
2. Prior Art
An outer electrode that is used as an terminal electrode or equipped in a surface pattern constituting a circuit in an electronic part which uses a magnetic or dielectric material as an element or in a laminated type electronic parts is formed by printing or coating a conductive paste, drying it for removing a solvent, followed by calcining. Further, for maintaining an adhesive strength between the outer electrode of the electronic part and a printed circuit board having the electrode mounted thereon, plating is carried out on a surface of the outer electrode and soldering is further conducted at the interface between the both.
Since the outer electrode is required to have a mechanical strength, the conductive paste which forms the electrode is formulated with glass frit in addition to conductive particles, resins and solvents, for the purpose of providing a mechanical strength to the electrode. In this case, when the temperature for calcining the conductive paste is high, there is a possibility of causing inner defect by melting of inner electrodes in an element. Therefore, the one with a low melting point containing a large amount of lead oxide has been conventionally used for the glass frit contained in the conductive paste for this use.
On the other hand, the use of the glass frit containing a large amount of lead oxide has a significant influence on the environment. Therefore, in terms of an environmental preservation, a conductive paste using glass frit containing substantially no lead oxide is earnestly desired.
However, if glass frit containing no lead oxide is used, an adhesive strength between the electrode and the element, the substrate, etc. is not attained, thereby creating a tendency of exfoliation of the electrode from the element or the substrate. This problem can be overcome by elevating the calcining temperature for the conductive paste, however, it causes melting of the inner electrode as mentioned above.
Accordingly, an object of the present invention is to provide a conductive paste for an outer electrode, which uses glass frit containing substantially no lead oxide, which can be calcined at a low temperature and which sufficiently provides an adhesive strength between the electrode and the element or the substrate. A further object of the present invention is to provide an outer electrode of an electronic part using the conductive paste and a process for producing the same, and to provide an electronic part using the electrode.
The present inventors have made extensive and intensive studies for solving the above problems and found a composition of glass frit satisfying the above-mentioned objects, and thus, the present invention has been completed.
Accordingly, the present invention relates to a conductive paste for an outer electrode of an electronic part comprising glass frit, a vehicle and conductive particles, wherein the glass frit comprises
(1) substantially no lead oxide; and
(2) as an oxide unit,
B2O3 in an amount of 5.0 to 30.0% by weight,
SiO2 in an amount of 10.0 to 60.0% by weight and at least one oxides selected from the group consisting of BaO, ZnO, Al2O3 and Na2O wherein a content of BaO is 60.0% by weight or less, a content of ZnO is 30.0% by weight or less, a content of Al2O3 is 12.0% by weight or less and a content of Na2O is 15.0% by weight or less;
The present invention further relates to an outer electrode of an electronic part which is obtained by calcining the conductive paste, to a process for producing the outer electrode of an electronic part which comprises the step of calcining the conductive paste at a temperature of 500 to 750xc2x0 C. and to an electronic part equipped with the outer electrode.